Simon Riggs <simon@2ndQuadrant.com> writes:
> On Tue, 2010-06-08 at 18:03 -0400, Robert Haas wrote:
>> OK, yes, I see what you're getting at now. There are two possible
>> ways to do freeze the tuples and keep the xmin: we can either rely on
>> the PD_ALL_VISIBLE page-level bit (as I previously proposed) or we can
>> additionally have a HEAP_XMIN_FROZEN bit as you propose here. I am
>> not sure which way is better.
> Doing it at tuple level is more flexible and allows more aggressive
> freezing. It also works better with existing tuple visibility code.
I agree, relying on a page-level bit (or field) is unpleasant in a
number of ways.
But none of this accomplishes a damn thing towards the original goal,
which was to avoid an extra disk write associated with freezing (not
to mention an extra write for setting the transaction-committed hint
bit). Setting a bit is no cheaper from that standpoint than changing
the xmin field.
regards, tom lane
